Blisters

Foot and Leg Pain

Blisters are caused by the skin
having to stretch too much (it's called skin shear) and not the
typical rubbing that we think would cause them.

When the skin stretches (shears) too far and for
too long, the connections between skin cells fatigue and break.
These tiny tears under the skin surface are the start of the blister
injury. Fluid fills the injured area and that is when it starts
to look like a blister.

Interesting fact: It can take up
to two hours for a blister to fully fill with fluid!

Relevance of heat, moisture and friction?

The other popular theory is that heat, moisture and friction
cause blisters. While these factors are relevant, they represent a shallow
and incomplete understanding of the blister process - and an unhelpful
one at that. Here's why:

Friction has an unfortunate double meaning: one is rubbing,
the other is the degree of grip or slipperiness. The latter definition
is the one we need to use.

It's how high friction levels contribute to skin shear
that causes blisters. High friction levels cause the skin, sock and shoe
to stick together for a bit longer. And because the bones continue to
move inside your foot, the skin is made to stretch.

Requirements for blister causing shear:

1. Type of skin

The skin of our feet is susceptible to blister formation, particularly
the plantar (sole) surface. That’s because it's thicker and less
mobile than other skin and most able to form and maintain a fluid-filled
lesion. And every individual's skin has an inherent shear strength. We're
all different in many ways, from blue eyes to big noses ... shear strength
is just another of those differences.

2. High coefficient of friction (pressure and friction)

No other body part sustains pressure like the foot does
- because we walk on them. And there are few other body parts where the
micro-climate dictates there will be high friction by default (heat, moisture
& little evaporation). This high coefficient of friction means the
skin grips the sock, and the sock grips the shoe. These surfaces remain
stuck together, for longer. The result is internal tissue layers slide
further relative to each one another. When the connections that bind these
layers are stretched too far, they tear (it's a mechanical fatigue - Comaish,
1973). Fluid fills the injury site and you have a blister.

Insight: To see how important friction is, put a drop
of oil on the back of your hand (minimise friction) and wobble it. Regardless
of how hard you press, there is almost no shear. Seriously ... do it now!
This is why minimising friction is key to blister prevention.

3. Moving bone

As you walk, run and play, the bones move around within
your foot. As your foot plants, the bones skid forward to a stop, and
then backwards as they push off into propulsion. This is normal and unavoidable.
But the further the bones move, and in the presence of high friction,
skin shear can become excessive. That is, the structural connections fatigue
and fail. This is a blister-causing level of skin shear.

4. Repetition

The more times soft tissue is subjected to these high
shear distortions, the closer the structural connections between the tissue
layers are to breaking and forming a blister. Everyone sits in a different
position on the blister continuum and this explains why some people seem
to be blister prone - they get blisters with relatively few repetitions.
And others are blister resistant - they can run day after day in the most
challenging of conditions and not suffer a single blister.